The present invention relates to an overvoltage protection circuit for a line amplifier fed remotely through the line.
Such overvoltage protection circuits are disclosed, for example, in "Technische Mitteilungen [Technical Reports] AEG-TELEFUNKEN", Annex: Analog Transmission Devices, 1977, on page 72, and in FIG. 1 on page 71, and in German Auslegeschrift [Published Application] No. 1,462,247. In these prior art protective circuits, overvoltage arresters are provided at each end of the line. These arresters conduct, or fire, when struck by lightning or under the influence of induction from power lines and thus protect the amplifier. However, the firing of the overvoltage arrestors produces steep voltage wave fronts which have high energy components over the entire transmission range of the line amplifiers and thus not only interfere with transmission but may even cause damage of the amplifier unless additional measures are provided.
German Offenlegungsschrift [Laid-open Application] No. 2,843,919 discloses a line amplifier including a circuit arrangement for providing coarse protection against overvoltages at the input and output, with an overvoltage arrester being provided only at the input of the line amplifier. This has the advantage that the above-mentioned steep voltage wave fronts during firing of the overvoltage arrester can act only on the amplifier input, which, however, is customarily provided with a pre-equalizer that strongly attenuates the lower transmission frequency range or weakly attenuates the medium transmission frequency ranege, respectively. But in this way only part of the pulse energy generated during firing of the overvoltage arrester, which energy is more or less distributed uniformly over the entire transmission range of the line amplifiers, is eliminated.
It is customary that protection of line amplifiers against overvoltages is set up in several steps for instance 2. In the latter case, the protection normally consists of coarse and fine protection, the difference being defined by different firing voltages.